Flex circuits are used to electrically couple components together such that relative motion between the components is made possible while maintaining electrical coupling. Relative motion between components can be desirable in either the assembly of the components or in their operation. If relative motion is provided by the flex circuit to facilitate assembly, the number of flex cycles can be as few as one or two flex cycles. If the relative motion provided by the flex circuit is required for the operation of the components, the flex cycles can be as much as several thousand to hundreds of millions of flex cycles.
Fatigue of a flex circuit is a concern for flex circuit designers and the users of flex circuits. The impact to a user of a flex circuit failing can vary from not being able to play music from a CD player to not being able to access data from a hard disk drive. The degree of impact to a user will depend on the device in which a flex circuit that fails is employed.
Fatigue of a flex circuit can occur when tensile stress is cyclically applied to the copper conductors, known as traces. The tensile stress can cause grains, and dislocations between the grains to move and align themselves such that a microscopic crack forms. Once a microscopic crack has formed, it can propagate under repeated tensile stress and grow into a separation in the trace and hence a crack that can prevent reliable electrical coupling.